Pyrotechnic compositions are used for may useful purposes. Such compositions are used in the aerospace industry to provide ignition, propulsion, vehicle separation, and emergency egress. The automotive industry also uses ignition and gas production compositions for occupant restraint systems, i.e. airbags. The demand for these pyrotechnic mixtures continues to grow.
A typical pyrotechnic device is the NASA standard initiator (NSI). The NSI is a two-pin electrically activated, electro-explosive device containing zirconium potassium perchlorate (ZPP) propellant. Initially designed and used in 1966 for the Apollo lunar mission, the standard NSI is still used today in the NASA space shuttle system.
The propellant used in the NSI is a composition of finely divided zirconium, potassium perchlorate and graphite held together with a fluoroelastomer polymer as a binder. The mixture, besides being highly reliable, is very sensitive to ignition stimulus, particularly static discharge. The propellant is an extremely fast brisant explosive. These properties make the manufacture and handling of bulk quantities of ZPP very hazardous.
In the automotive industry, inflatable vehicle occupant restraint systems are standard equipment included in millions of new vehicles every year. Although different propellants are sometimes used in automotive applications, the initiators are very similar to the NSI's in that they usually use a ZPP propellant and are generally activated by electrical stimulation. The manufacture of propellants has been an inherently dangerous task owing to the risk of fire or explosion. The pyrotechnics industry has experienced accidents in the manufacture of energetic compositions, particularly with hand blending techniques. Flare and illumination compositions based on metal fuels such as, for example titanium, aluminum, magnesium and the like, exhibit characteristics similar to ZPP.
The performance of an energetic material such as ZPP is dependent on many factors such as purity, particle size distribution, particle shape, surface area, and the like. One of the factors affecting the reproducibility of the performance of a propellant is the degree of mixing, or homogeneity of the blend. A pyrotechnic composition that has been poorly mixed often exhibits slower burn rates and is less dependable than a well-mixed one. Some pyrotechnic compositions make use of a binder system that serves as an adhesive, holding the fuel and oxidant in a well-mixed condition. Without a binder many compositions separate under the influence of gravity or vibration, resulting in performance degradation. Therefore, proper mixing and incorporation of the binder during manufacture are key process parameters.
Heretofore, the blending of the NSI propellant ingredients has been done by hand, typically using a solvent evaporation procedure. The propellant components were generally poured in the form of dry powders into a 45 degree inclined bowl rotating with a solution of the binder (fluoropolymer in acetone or n-butyl acetate, for example). Manipulating the blend for homogeneity, the solvent was evaporated to leave the binder and obtain a moist solid. The moist solid was then sieved in air through a screen, often by hand. The screened propellant composition was then dried to remove the residual solvent. This process has produced good propellant, but has a number of disadvantages. The solvent evaporation technique relies on manual and frequent movement of the mix during processing to achieve good blends. Thus, the outcome of the blend is very dependent on the skill of the person doing the blending. The solvent evaporation method can also be very dangerous. Fire and explosion have occurred during the solvent evaporation blending process. The operator is in close proximity to the mixing and granulation process, a high risk situation. The evaporation method is also time intensive, requiring several hours per blend.
The preparation of propellants has also been done using a precipitation technique. In this method, the fuel and oxidizer components are suspended in the binder solution by a mechanical mixer and a countersolvent is added while mixing the solution. The countersolvent causes the binder to precipitate from the solvent. As the binder precipitates, the active particles are entrapped in the binder. This process has historically been a manual operation with the operator in close proximity to the mix container, adding the dry components and countersolvent. The timing of process events lacked a degree of repeatability due to the human operator. Previous attempts to use the precipitation methodology for the preparation of propellants resulted in poor repeatability. These blends have suffered from rubbery inclusions, or the formation of clay-like products that require granulation similar to the evaporation process. Fire and explosion have also occurred during the precipitation blending process. These incidents have usually been due to pyrophoric or static discharge ignition of the metal fuel component. The operator has also been at risk in close proximity to the mixing process which involves the use of volatile, flammable solvents, as well as the propellant particles.
From the descriptions above it is evident that the largest factor reducing personnel safety is close proximity to the blending operation. The ability to perform the blending operation from a remote location would greatly increase safety. It would be desirable to have an automated propellant blending system in which the quality and characteristics of the propellant composition are not so dependent upon the skill of the human blending operator. It would also be desirable to speed up production and expand the capacity of propellant manufacturing facilities. It would be further desirable to have available a propellant blending system which avoids the evaporation method and the safety hazards incidental to drying the propellant composition to dryness. It would also be desirable to use a propellant blending process which minimizes the screening requirement and obtains a finer end product than has been available heretofore. Ideally, an automated propellant blending system would be able to produce agglomerated propellant particles having a controlled particle size distribution.